Dr. Mac Cull och on the Parallel Roads of Glen Roy. 337 



words, will render easily intelligible. There is no need for the 

 pickaxe and spade to investigate this question, since the water 

 courses have produced innumerable natural sections ; recent ones 

 may be found after every fall of rain, a circumstance in which 

 Lochaber is by no means deficient. 



The extreme breadth of these lines may safely be taken at seventy 

 feet, or a little more, and their most general one lies between that 

 and fifty. As in no instance that I have remarked do they exceed 

 the former, so they very rarely indeed fall short of the latter dimen- 

 sion. The most remarkable exception to this rule has been already 

 noticed in describing the upper part of the glen ; and it may not 

 be amiss to repeat that the lines are narrowest and least marked 

 on the hardest and most rocky ground, where in fact they cannot, 

 with any latitude of language, be called roads, since they are abso- 

 lutely invisible to a person when standing on them. In no case is 

 their surface level, but it lies at various angles with the horizon, 

 from 30° and upwards to 20° and 12°.* It is probably from this 

 cause that they are in many cases invisible where we should other- 

 wise expect to find them ; their own inclination coinciding so 

 nearly with the general slope of the ground as to render them im- 

 perceptible from the place of the spectator.")" Both the interior and 

 exterior angles are very much rounded ; % and the surface, I need 

 scarcely add, is marked by considerable inequalities, from the fall 

 of stones, and the partial accumulation of plants and recent soil. 

 In describing their relation to the side of the hill, they may be said 

 to bear the resemblance of sections of parallel layers applied in suc- 

 cession to its face.§ In only one instance is there a slope, resembling 

 a superior talus, and this is visible for perhaps half a mile j^f while 



*Vide Profiles PI. 18. + Prof. 1, 5. % Prof. 8. § Prof. 7, q Prof. 6. 



